Subterranean, geologic formations may include permeable strata or layers, such as sand, which are generally bounded on at least one side by an impermeable strata, such as shale. The permeable strata have the capacity for holding liquid and gaseous fluids which may flow through the strata within the bounds of the impermeable strata. In relatively shallow formations, permeable strata may comprise aquifers and sources of useful water. In relatively deep formations, permeable strata may comprise reservoirs of brine or hydrocarbons such as oil and natural gas.
A permeable subterranean formation bounded both above and below by impermeable strata may have the capacity to receive and hold fluids or slurries and, as such, be attractive as a repository for the permanent storage of hazardous waste materials, such as radioactive liquids or slurries, waste solids or sludges, or other toxic or undesirable liquids or gasses.
To remove hydrocarbons from, or to inject waste materials into, the subterranean formations, wells, such as oil wells, are formed which extend from a well head at the ground surface to the subterranean formations.
Whether fluids or slurries are removed from or injected into a permeable subterranean formation, it is often desirable to fracture the permeable formation to enhance the flow rate of fluids from, or into, the permeable formation. The fracture of the permeable formation is commonly achieved by injecting an incompressible fluid in a sufficient volume at a sufficient hydraulic pressure via the well into the permeable formation to cause the permeable formation to part, or fracture, under the transmitted stress.
In many instances, one or more upper permeable formations may be located above other lower permeable formations, with the permeable formations being separated by at least one impermeable formation. Commonly in such situations, the shallow or upper formation may be an aquifer and carry fresh water, useable as drinking water, and the deeper or lower formations may contain oil, gas, and/or brine, or may have the capacity to receive and hold waste materials. If one or more of the lower formations is fractured to inject wastes, the fracture must be controlled so that the fracture does not extend completely through the impermeable zone and permit hydrocarbons or waste materials to pass through from the lower formation to the upper formation. Such a fracture could result in a complete failure of the fracture treatment, contamination of the upper formation with the fracturing fluid and/or fluids from the lower formation, and, in the case of an oil or gas well, poor oil and/or gas production, the requirement of further remediation, or even abandonment of the producing zone. If waste material is to be injected into the lower zone, then a total fracture through the impermeable zone could pose serious and even illegal environmental hazards.
Conventionally, the upward growth of a fracture is monitored using temperature logs, tracer logs, or seismographs. Temperature logs and tracer logs, however, are generally unreliable and are used after-the-fact rather than in real time, thereby posing a potentially unacceptable hazard wherein hydrocarbons or waste products could leak into water that may be useable for human consumption. Seismographs are relatively expensive and require real-time data interpretation and display. There are thus many drawbacks associated with using conventional techniques for detecting the upward growth of a fracture.
Therefore, what is needed is a reliable, economical system and/or method for detecting, in real time, the upward growth of a hydraulic fracture.